A turbine is a device connected to a shaft and by means of which the energy from a working fluid can be transferred to the shaft. Amongst different types of turbines, a radial turbine is a turbine where the flow enters a radial direction and is turned in the rotor passage to exit in the axial direction. In a mixed-flow turbine, the flow enters with both a radial and an axial component, but usually primarily in a radial direction. Such a feature of radial and mixed-flow turbines makes it suitable for applications where a compact power source is required. The main applications can be divided into three main areas: automotive, aerospace, marine, power generation and other suitable energy recovery applications where a radial turbine is usually part of a turbocharger.
Turbocharging is the most common way of supercharging a reciprocating internal combustion engine since turbochargers are smaller in size, lighter and cheaper than other available devices. The principal aim of supercharging an internal combustion engine is to improve the power density. Supercharging can be defined as the introduction of air (or air/fuel mixture) into an engine cylinder at a density greater than ambient. In doing this, a greater quantity of fuel can be burned in one engine cycle with a consequent rise in the power output. In turbocharger applications such an increase in power output is achieved by using the exhaust gases generated by combustion to power the turbine and in turn the compressor is powered. By doing this the energy of the exhaust gases which would be wasted is then recovered.
A turbocharger is constituted by three main elements: compressor, bearing housing and turbine. A typical turbocharger design is shown in FIG. 1. The turbocharger has a compressor scroll (CS), an impeller (I), a shaft (S), a turbine volute (TH), and a turbine wheel (W). The working scheme of a turbocharger is shown in FIG. 2, in which is shown the engine (E), an intake manifold (IM), an exhaust manifold (EM), a turbine (T), a compressor (C) and a shaft (S). As the exhaust gases quickly move out of the engine cylinders (E) and flow into the exhaust manifold (EM), they are directed into the turbine (T). As the gases flow through the turbine housing (TH), they come in contact with the turbine wheel (W). As they flow through this airflow path and into the exhaust down pipe, they spin the turbine wheel, imparting a portion of their kinetic energy to the turbocharger. By the connecting shaft (S) the power gained in the expansion process is transferred to the compressor (C) which compresses the incoming air through the impeller (I). The compressed air then flows into the compressor scroll (CS) where further compression can take place and finally will be squeezed into the engine cylinders through the intake manifold (IM). After being expanded in the turbine, the exhaust gases leaving the turbine are usually directed into the tail pipe and then expelled to the ambient environment. However the exhaust gases leaving the turbine still have some energy which could still be extracted to further enhance engine performance. Using a further device to accomplish this task is usually referred as “turbocompounding”.
Unlike turbochargers (for which the energy extracted from the exhaust gases is directly transferred to the compressor) a turbocompound unit is constituted by an exhaust driven turbine which transfers the energy recovered by the exhaust gases directly to the crankshaft (mechanical turbocompounding) or to an electric generator feeding a battery (electric turbocompounding) via the shaft. Nevertheless it should be understood that the pressure from the exhaust gases available to the turbocompound unit is not large since most of the expansion has already occurred in the turbocharger turbine. The turbocompound unit must be able to operate at very low pressure ratios, for example, with an inlet to outlet pressure ratio of between approximately 1.02 and 1.2. Radial and mixed-flow turbines currently available in the market are designed to operate at higher pressure ratios for which they usually provide a peak normalised total-to-static efficiency which ranges from 0.9 to 1.0. This is shown in FIG. 3 where a typical turbine map for a conventional turbocharger turbine is presented. From FIG. 3 it can be seen that in the pressure ratio (PR) regions greater than 1.2, the turbine performance is as large as ≈0.9. However, as soon as the pressure ratio drops below 1.2, the turbine normalised total-to-static efficiency falls dramatically to values below 0.6. Such a trend is common to all radial and mixed-flow turbines currently existing in the market. As a turbine with normalised total-to-static efficiency below 0.6 is not suitable for use in energy recovery applications, existing turbines are not suitable for use in turbocompounding at low pressure ratios.
Thus it is an object of the present invention to address this deficiency in the prior art technology.